What size needle do you use for machine piecing? I have read to use a 9, someone else told me 11. Some of the info is confusing. Is it an individual preference? I have a simple Brother machine, if that makes a difference.

the needle size is dependant on the thread being used-
visit a thread site (like Superior threads) they have free charts you can print off that tells you exactly what needles to use for what threads/weights/types-and what each needle is good for.